I want to make inset face planar
Keyboard Period . > Set pivot to 'Active Element'
Select center face, (the one to be moved,) Header 'Transform Orientations' dropdown > '+' create a new transform orientation from it
Ctrl Numpad + expand the selection to include the rim.
Keyboard 1 go to Vertex mode 
Shift LMB twice, Deselect and select one of the rim vertices to make it active
S Z 0